Comparing three common nest survey methods, using double-crested cormorants as a proposed sentinel for monitoring plastic pollution in freshwater environments

Many bird species use plastic as nest-building material, and nest surveys represent a unique opportunity to monitor environmental plastic pollution. However, the current literature lacks consistent, repeatable methodologies, making comparison across studies challenging. This study evaluated three common nest survey methods to assess the use of nest debris: photographic assessment, visual assessment, and nest deconstruction. We applied these methods to double-crested cormorant nests ( Nannopterum auritum ) at two locations on Lake Ontario, Canada. We found that for this species, nest deconstruction yielded the greatest accuracy and detail for detecting debris abundance, type, and colour. Of the two non-invasive methods, visual surveys outperformed photographic surveys across all metrics assessed. In our mainland colony, 100% of nests contained debris based on visual assessment, while at our offshore colony only 37.4% of nests contained debris, suggesting that location influences nest debris for this species. We detected Personal Protective Equipment (PPE)-related debris in 20% of all nests across survey years at our mainland colony (2022 and 2023), representing the first time that PPE has been documented in the nests of this species and demonstrating that nest surveys can be a useful tool for capturing changes in an evolving pollution landscape. This study contributes evidence that double-crested cormorants may be a useful sentinel species for monitoring plastic pollution in understudied, freshwater environments. Our results also demonstrate that the chosen method can greatly impact the results of a nest survey, and careful consideration of methodology should be undertaken before implementing a nest survey to monitor plastic pollution.